GMAT Sentence Correction (SC) | Re: SC5

rather than concentrating on the idioms list lets try the elimination method.
2/3 split:

its either 'but' or 'while'.

so, here we are comparing an industry's rapid growth with debt levels..quite opposite in nature...so we will go with ' BUT'. eliminate A and B.

In D, 'proving' is continuous tense , the company has already lost money in debt..so eliminate D.

Between C and E, the "it" in option C refers to a null entity. So we go for E.

Hope it helped.